The present invention relates to a method for the anticipatory speed control of electric drives, in which the speed control is effective over a plurality of blocks.
In modern industrial controllers that are used in machine tools or robots, for example, a feed rate for the speed control of associated electric drives is generally preset block by block in subprograms. In order for the electric drives to operate at a speed that is consistent with the programmed feed rate, an actual feed rate must be able to be reduced in a timely manner. Unless the actual feed rate is reduced within a relatively short time, especially when block download disturbances occur, the electric drive will not be able to stop within a reasonable time from a high feed rate speed. The same problem also occurs when the programmed feed rate of a following data block of the subprogram calls for a reduction in the feed rate that is considerably lower than the current travelled feed rate speed. In this situation, because of the restricted acceleration capabilities of the electric drive, a proper lowering of the currently travelled feed rate speed to the programmed speed of the following block is no longer possible.
This problem of lowering the feed rate is further exacerbated when a feed rate override is provided in order to influence the speed control of the electric drive, especially when the electric drive has a braking path that is proportional to the square of the increase in speed (i.e., the braking path is multiplied fourfold in the case of a doubling of the speed). Such an override permits the programmed absolute feed rate to be varied proportionally in the range from 0 to, for example, 200%, in order to be able to adapt to changed technological circumstances during a machining process. When this feed rate override is used in conjunction with an electric drive having such a braking path relationship, the problem of lowering the current feed rate speed to the speed of the following block becomes exacerbated. Assuming that an intended speed profile is provided with 100% override, and that a reduction of the current feed rate to a lower programmed speed value of the following block is possible within the current block length, it would be impossible to achieve a proper braking if an increase in the override value and the associated quadratic extension of the braking path is implemented. Since the consequence of taking into account such override values over a plurality of blocks is a high computing requirement, it has been attempted to configure a speed control in such a way that a controller which is effective over a plurality of blocks operates rapidly and reliably, even in any override range.
The above-described conventional speed control methods are effective over a plurality of blocks and take into account any override value possibly present. Such conventional methods suffer from at least one of the following drawbacks: either they consider only a single subsequent block (in this regard, see the Siemens SINUMERIK 840 C controller) or else, in the block preamble, they take into account only the preset programmed intended speed profile at 100% override. Hence, on the one hand, anticipatory braking over a plurality of blocks is not possible, or else, in the second case, the override range which it is practical to cover is very severely restricted. Both methods, which are used in accordance with standard procedure, thus have the disadvantage that the efficiency of the electric drive used can generally be utilized only to a limited extent.